This invention relates to an improvement in the known process for removing nitrogen oxides and sulfur dioxide from waste gas by scrubbing with a slurry of ferrous sulfide (FeS). The SO.sub.2 is captured according to the following chemical reaction: EQU 3FeS+2SO.sub.2 .fwdarw.FeSO.sub.4 +2FeS.sub.2.
The nitrogen oxides are converted to N.sub.2 by reactions involvingFeSO.sub.4 and FeS. The conversion of the nitrogen oxide to N.sub.2 increases the FeSO.sub.4 production in the scrubber.
The insoluble FeS.sub.2 and a portion of the soluble FeSO.sub.4 present in a wet filter cake are thermally regenerated in the presence of a reducing agent to produce FeS for reuse in the scrubber. Ideally the sulfur is recovered from the regenerator as elemental sulfur. This process is described in detail in U.S. Pat. No. 4,130,628 entitled "Process for Removing SO.sub.2 and NO.sub.x From Gases." This reference is incorporated herein by reference to the extent that it supplements the disclosure of the improved process disclosed herein.
It has now been discovered that the thermal conversion of iron sulfate (FeSO.sub.4) to iron sulfide (FeS) does not occur efficiently in the thermal regeneration step. In the regenerator FeS is ideally produced by the following reaction: EQU FeS.sub.2 .fwdarw.FeS+S EQU FeSO.sub.4 +reductant.fwdarw.FeS+gaseous oxide.
What reactions actually take place in the regeneration are not easily determined as it is not an easily controlled and studied system. Apparently, a side reaction occurs between the FeSO.sub.4 and the newly formed FeS that further converts FeS to undesirable iron oxide and sulfur dioxide. See, for example, "Thermal Decomposition of Iron Sulphates II" Huuska et al., J. appl. Chem. Biotechnol. 1976, 26, 729-734. The side reaction may be as follows: EQU FeS+5FeSO.sub.4 .fwdarw.2Fe.sub.3 O.sub.4 +6SO.sub.2 .uparw.
The released SO.sub.2 must be recaptured in the scrubber reducing the overall efficiency of the system. The iron oxide must be discarded. Unfortunately, the iron oxide is intermixed with FeS being returned to the scrubber and cannot be separated. Eventually intermixed FeS and Fe.sub.3 O.sub.4 must be discarded and the process started over with pure FeS.
Better separation of the FeS.sub.2 containing slurry from the soluble FeSO.sub.4 in the wet filter cake does not help. Washing the filter cake would produce an FeSO.sub.4 liquor which would have to be purged from the system causing irretrievable loss of its FeS value.
The loss of recyclable products according to the above-described process is 331/3 to 40% depending upon whether FeSO.sub.4 is washed from the filter cake or allowed to react with FeS during thermal recovery to form inert Fe.sub.3 O.sub.4. The loss is even greater if the quantity of nitrogen oxide being scrubbed increases thus producing quantities of FeSO.sub.4.